Fatigue experiments under constant torsional strain were carried out with an Al-4% Cu alloy. The energy loss △E was found to decrease to about zero in the initial stage of experiment, and to rise again after a certain number of cyclic loading. Results of metallographic observation showed that corresponding to the up-rise of △E, the slip regions became suddenly localized. These observations confirmed our previous assumption that the appearance of localized coarse slip regions is one of the basic processes giving rise to △E. Under fatigue loading, the maximum torque Tm was found to rise in the initial stage of experiment. It reached a maximum value and maintained there for a certain number of cyclic loading, after which a sudden drop occurred, and this drop corresponded to the up-rise of △E. Such an occurrence seems to be an indication of the sudden un-pinning of those dislocations which were pinned during the initial stage of fatigue loading. It is contemplated that the process of pinning of dislocations may be due to the nucleation of clusters of copper solute atoms drawn from the Cottrell atmosphere formed during the initial stage of experiment, along certain favorable sections of the dislocations, in consequence of which other sections of the dislocations were subsequently unpinned from copper solute atoms. As the clusters grow up in size, the length of the un-pinned dislocation segments increases until a certain critical value is reached, at which an avalanche occurs and numerous dislocations are produced. Such a happening results in a sudden drop of Tm and leads to the formation of localized slip regions which give rise to an up-rise of △E.